1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method of forming an orientation film of a liquid crystal display (LCD) device.
2. Description of Related Art
A typical LCD device includes lower and upper substrates with a liquid crystal layer interposed therebetween. The lower substrate has a thin film transistor (TFT) as a switching element and a pixel electrode, and the upper substrate has a color filter and a common electrode. The pixel electrode serves to apply a voltage to the liquid crystal layer along with the common electrode, and the color filter serves to implement natural colors.
A manufacturing process of the LCD device is explained below. First, the lower substrate is prepared. On the lower substrate, a plurality of thin film transistors and pixel electrodes are arranged in a one-to-one arrangement. To form the orientation film on the lower substrate, the surface of the lower substrate is coated with an organic film such as a polyimide, and the orientation film is rubbed in a direction. It is desirable that the orientation film is formed to keep a uniform thickness of the whole surface over the lower substrate. The rubbing is performed normally by using a cloth. Aligned on the orientation film, the liquid crystal molecules orient themselves uniformly in the direction of the rubbing. By the rubbing treatment of the orientation film, liquid crystals can be driven normally, and uniform display characteristics can be obtained. Then, sealant is applied in a picture-frame-like pattern, either by means of screen-printing or dispensing. Sealant is required in order to make a liquid crystal cell from two substrates. In addition to sealing the liquid crystal, sealant protects the liquid crystal from contamination from external sources such as the penetration of water, and from environmental changes. Next, spacers are sprayed to keep a uniform cell gap between the lower and upper substrates. Important requirements are producing uniform spraying over the entire substrate, controlling the spacer density (number of spacers per unit area), and preventing the formation of lumps. The spraying technique includes a wet method of spraying spacers and a dry method of spraying spacers. The wet method of spraying spacers involves the following steps: diffusing spacers in a low-boiling-point organic solvent, such as freon or alcohol, through the use of ultrasonic waves; spraying the spacer-diffused liquid; and drying the panel in order to evaporate the solvent. In the dry method, spacers are applied electrostatically or diffused by means of an air-jet. When TFT-LCDs are handled, measures should be taken to prevent a static electricity. In addition, freon, alcohol, and other organic solvents are subject to density control. Because of this, the dry or air-jet diffusion method is mainly used. Sequentially, two substrates are assembled with and attached to each other. Alignment of the two substrates depends on alignment error, can be several micrometers. When the two substrates are aligned with each other beyond alignment error, since light leakage may occur, desirable display characteristics cannot be obtained. The next step is a cell-cutting process. The liquid crystal cell manufactured through the foregoing five steps is cut into a unit cell. The liquid crystal cell undergoes the cell-cutting process after a plurality of liquid crystal cells is formed on the large-sized glass substrate. The cell-cutting process includes a scribing process that forms a cutting line on a surface of the substrate with a pen of a diamond having a higher hardness than the glass substrate, and a breaking process that breaks the liquid crystal cell into unit cells. Then, a liquid crystal is injected into the gap between the two substrates. The unit liquid crystal cell generally has an area of hundreds of cm2 and a gap of several micrometers (μm). A vacuum injecting method, which uses a pressure difference between inside and outside of the cell fills the liquid crystal between the two substrates, for example.
At this point, the orientation film is essential to the LCD device. The LCD device displays images by using the dielectric constant anisotropy of liquid crystal molecules and by changing their orientation through the application of a voltage. A critical element of the LCD device is the ability to control the orientation and the pretilt angle of the liquid crystal molecules for a given operating mode. The orientation film is designated to perform these functions.
FIG. 1 shows a conventional method of forming the orientation film. As shown in FIG. 1, an equipment of coating an orientation film includes a doctor roll 11, an anilox roll 13, a printing roll 15, and a rubber pad 17 attached on the printing roll 15. The doctor roll 11 is geared with the anilox roll 13, and the anilox roll 13 is also geared with the printing roll 15 for rotation. The anilox roll 13 has fine grooves 13a on its surface. The rubber pad 17 has embossment patterns (⊥) on its one surface.
In order to form the orientation film, first, an array substrate 10 is provided on a stage 12. When the coating equipment is operated, the doctor roll 11 rotates with the anilox roll 13 geared therewith, spraying an orientation material. The sprayed orientation material is deposited into the fine grooves of the anilox roll 13 Subsequently, an orientation material deposited in the grooves of the anilox roll 13 is transferred to the rubber pad 17 of the printing roll 15, with the anilox roll 13 rotating with the printing roll 15 geared therewith. Thereafter, an orientation material transferred to the rubber pad 17 is coated on the glass substrate 10 in the form of the patterns (⊥) of the rubber pad 17 while the printing roll 15 rotates. The orientation material is coated on rest regions of the substrate other than a sealant region and a pad region.
However, the conventional method of forming the orientation film requires preparatory proceedings including attaching the rubber pad to the printing roll and cleaning the rolls. If the cleaning process is not cleanly carried out, owing to contamination, spots may take place on the substrate on which the orientation film is formed, or pinholes may occur during a hardening process, thereby lowering manufacturing yields. Further, since the anilox roll 13 has the fine grooves on its surface, the anilox roll 13 is badly worn if it is used during a certain period, so that a shape of the fine grooves may be transformed. Therefore, after a certain period, the doctor roll 11 and the anilox roll 13 should be replaced. Therefore, during replacement of the doctor roll 11 and the anilox roll 13 subsequent processes cannot be performed. Besides, since the rubber roll 17 is lower in hardness than other components of the coating equipment, the life span of the rubber pad is short and also its emboss patterns should be changed according to the model. Therefore, operation efficiency becomes lowered and a production cost also becomes high.